Optical networks using Remotely configurable Optical Add/Drop Multiplexers (ROADMs) or Photonic Cross Connects (PXCs) to optically route are commonly deployed in optical networks, and provide the benefits of lower cost, greater flexibility in data formats, and hitless capacity upgrades. In such networks, the signals may remain in the optical domain for thousands of km, passing through many optical amplifiers (OAs) and multiple ROADMs/PXCs (nodes). To operate such a network successfully on a large scale, a new suite of management tools that support dynamic wavelength routing is needed. Such tools should promote no-touch provisioning, adaptive fault tolerance, and intelligent performance monitoring with prediction of impending failures, while operating reliably in a challenging environment containing sparse Optical-Electrical-Optical (O-E-O) locations. To achieve this goal, many network parameters need to be monitored. One of the network parameters that needs to be monitored is referred to in the art as the “lightpath,” which is defined as the path followed by a particular wavelength from its source node, through various ROADMs and PXCs, and ultimately to the terminating node. Conventional wavelength-based monitoring methods cannot guarantee proper wavelength routing as these cannot distinguish optical signals with identical wavelengths that emanate from different source nodes such as is shown in the illustrative wavelength routing network 100, in FIG. 1.
Referring now to FIG. 1, there is shown a plurality of nodes, A 102, B 104, C 106 and D 108, and a photonic crossconnect (PXC) 110 for wavelength routing. Four signals (collectively labeled 112) originating at node 102 and four signals (collectively labeled 114) originating at node 104 are transmitted to node 108 (signals now collectively labeled 118) and node 106 (signals now collectively labeled 116), respectively, through multiple optical amplifiers (OAs) 120.
To provide lightpath tracing capability, two different approaches have been proposed. The first is referred to as a “pilot tone technique.” For this method, an overlay characteristic pilot tone frequency is introduced for each optical signal, and lightpath tracing is achieved by monitoring the pilot frequency through low-frequency electrical spectrum analysis without using a wavelength selector (i.e. the total optical power is detected). This method has the advantages of modulation-format transparency and simplicity (for amplitude-based pilot tone), but it also suffers serious drawbacks. For example, amplitude-modulation based pilot tone methods suffer from Stimulated Raman Scattering (SRS) crosstalk. Phase/frequency pilot tone and polarization pilot tones are inherently immune to first order SRS crosstalk. These expedients are much more expensive to implement than amplitude pilot tone techniques since each signal requires an independent phase/frequency or polarization modulator, and each corresponding receiver requires a phase/frequency or polarization discriminator.
Another known lightpath tracing technique is referred to as “digital lightpath labeling.” This method introduces an overhead to encode the light label information, where the overhead varies the distribution of “1” and ‘0’ bits. In this regard, the digital label can be received by detecting the total optical power with a low-speed photodetector. This method can be easily implemented using intensity-modulation based optical communications, but it has not been demonstrated in next generation phase-modulation based (such as Differential Phase-Shift Keying) optical communication systems. In addition, this method also suffers from deleterious SRS crosstalk.
In view of the foregoing, a need exists for a new method for monitoring lightpath and other important network parameters, specifically for phase-modulation based optical communication systems.